His height obviously makes bending for low balls difficult, and reaching out to hit balls tailing way from him even tougher. And like a lot of two-handed power baseliners, del Potro feeds on pace.

A shift in the past 15 years has, arguably, transformed the two-hander into the most important live stroke in tennis.

How can something so beautiful to watch, a stroke so etched into tennis history, be so exploitable — and why have a dwindling handful of players remained loyal to it? By Matthew Futterman Reporting ...